Why does my Mazda RX-7 FD still run without the fuel pump relay?

If your FD RX-7 continues to run after disconnecting the fuel pump relay, there are two primary technical reasons this can happen:

1. You Pulled the Speed Relay Instead of the Main Relay The FD RX-7 utilizes two distinct relays for the fuel system. There is the Main Fuel Pump Relay (also known as the Circuit Opening Relay) located next to the EGI relay, and a separate Fuel Pump Speed Relay located at the front of the car ⁸.

If you removed the Fuel Pump Speed Relay, it does not cut power to the pump; it simply switches the pump's operating voltage ¹². To completely cut power to the pump, you must remove the Main Fuel Pump Relay or unplug the harness directly at the fuel pump hanger.

2. The Engine is Running on Residual Fuel Pressure If you successfully cut power to the pump but the engine continues running, it is operating on pressurized fuel remaining in the lines. According to the factory service manual (Page F-96, "Fuel Pressure Hold" test), the FD fuel system is designed to retain at least 21 psi for 5 minutes after the pump shuts off ¹².

Normally, the engine will only run for a few seconds on residual pressure before dying. However, if the engine runs for an extended period (e.g., up to a minute or two), it indicates excessive fuel pressure in the system. A faulty or gummed-up Fuel Pressure Regulator (FPR) can cause the system pressure to spike well above normal operating levels, meaning it takes significantly longer for the engine to bleed off that excess pressure ¹². As the fuel pressure slowly drops and the mixture leans out, you may also notice the idle RPMs climb before the engine eventually stalls.

Diagnostic Steps (Difficulty: Easy to Moderate)

• Verify the Relay: Double-check that you pulled the Main Fuel Pump (Circuit) Relay rather than the Speed Relay.
• Check Fuel Pressure: Connect a fuel pressure gauge to the system. With the engine off, check if the pressure is abnormally high (e.g., 45 psi). Apply vacuum to the FPR with a hand pump; if it fails to hold vacuum for more than a few seconds, the FPR is faulty and needs replacement.